EN
The paper reviews molecular markers and determinants associated with virulence, host adaptation and drug resistance in avian influenza viruses (AIV). The virulence of AIV is mostly dependant on the presence of multiple amino acids (mainly arginine and lysine) at the cleavage site of the haemagglutinin (HA) protein. The major factors contributing to host adaptation are also harbored within the HA protein: amino acids at positions 226 and 228 determine virus binding affinity to receptors present in cell membranes of birds or humans. It has been shown that pathogenicity and host adaptation are also dependant on the amino acid sequences of the polymerase complex (PB2-PB1-PA) and the most significant mutation (E627K in PB2) is related to the increased replication of the virus in mammalian cells. Molecular markers associated with an increased resistance to antiviral drugs are localized in neuraminidase (NA) and matrix (M) proteins. For example, a histidine to tyrosine substitution at position 274 of NA (H274Y) decreases viral susceptibility to neuraminidase inhibitors (e.g. oseltamivir), the most frequently used drugs in flu treatment. Monitoring of the molecular changes in the viral genome of AIV is very important from an epidemiological point of view and can be a valuable part of an early warning system.